By way of background, a gas turbine combustor is essentially a device used for mixing large quantities of fuel and air and burning the resulting mixture. Typically, the gas turbine compressor pressurizes inlet air, which is then turned in direction or reverse flowed to the combustor where it is used to cool the combustor and also to provide air to the combustion process. The assignee of this invention utilizes multiple combustion chamber assemblies in its heavy duty gas turbines to achieve reliable and efficient turbine operation. Each combustion chamber assembly comprises a cylindrical combustor, a fuel injection system, and a transition piece that guides the flow of the hot gas from the combustor to the inlet of the turbine section. Gas turbines for which the present fuel nozzle design is to be utilized may include six, ten, fourteen, or eighteen combustors arranged in a circular array about the turbine rotor axis.
In an effort to reduce the amount of NOx in the exhaust gas of the gas turbine, fuel nozzles have been developed that substantially premix air and fuel prior to the combustion flame, such that the temperature at the flame is reduced relative to conventional diffusion flames. Normal operation of these premixing fuel nozzles requires that a flame be prevented from forming within the premixing chamber. Moreover, the premixing fuel nozzles are designed to be able to eject and extinguish a flame that may inadvertently form in the premixing chamber due to momentary upset conditions owing to, e.g., a sudden transient in the gas turbine or a momentary change in fuel supply conditions.
Typically, the premixing chamber is not designed to endure the high temperatures encountered in the combustion chamber. However, a problem exists in that the combustor can be unintentionally operated so as to cause the flame to “flashback” from the burning chamber into the premixing chamber where the flame may continue to burn—a condition referred to as flameholding. Another problem that can lead to flameholding is the exposure of hydrogen or higher order hydrocarbons to gas turbines having premixing zones designed to normally run natural gas fuels. The presence of these components promotes flame speeds that are higher than methane and creates an environment where flashback is more possible and flameholding is more difficult to extinguish by the normal thermodynamics of a premixing zone designed to operate on methane. In either case, flashback and flameholding can each result in serious damage to combustor components from burning, as well as damage to the hot gas path of the turbine when burned combustor pads are liberated and passed through the turbine section.
U.S. Pat. No. 5,685,139 describes a premix nozzle that uses fuse regions near the discharge end of the nozzle to address flashback. In the event of a combustion flashback, these fuse regions burn through due to the higher temperatures experienced when the flame attaches to the nozzle's radial fuel injectors. The burn through allows fuel to substantially bypass the radial fuel injectors and thereby terminate the flameholding event. Any molten metal released into the combustor by reason of the rupturing fuse regions will be substantially vaporized in the combustion chamber without further damage to the combustor or hot gas path. Simultaneously, the combustor switches over from a premix burning mode to a diffusion burning mode until repairs can be effected. While the turbine will now operate with higher NOx emissions, it will nevertheless operate satisfactorily, with minimum damage to the combustor and no damage to the turbine itself.